Drilling fluids are one of the important guarantee measures that ensure safe, high-quality, efficient and rapid drilling production operation, and in oil field exploitation operation drilling fluids are needed.
Effective reservoir protection is an important guarantee for a high rate of oil and gas recovery, so as to attain an optimal negative skin coefficient. If the drilling fluid is designed or used improperly, the liquid and solid in the drilling fluid may intrude into the reservoir and have physical and chemical reactions with the clay and other minerals in the reservoir in the drilling process, causing severely decreased reservoir permeability in the immediate vicinity of the well and increased resistance against oil and gas flow to the bottom of the well and thereby resulting in a decreased oil production.
Generally speaking, the permeability of a low-permeability reservoir is 10-100 mD (millidarcy); therefore protectants designed for medium-permeability, high-permeability and super-high-permeability reservoirs are not suitable for protection of low-permeability reservoirs. Drilling in low-permeability reservoirs encounters various damages, for example reservoir damage resulted from clogging of solid particles in the drilling fluid, reservoir damage resulted from hydrated swelling and dispersed migration of the clay and minerals and reservoir damage resulted from water blocking etc.
Field tests have demonstrated that the above-mentioned damages to low-permeability reservoirs are difficult to solve with existing reservoir protecting agents and drilling fluids. It is extremely important to develop new reservoir protecting agents and drilling fluids for reservoir protection which are suitable for use under the present complex geological conditions. Up to now through long-time research on reservoir protective drilling fluids, treating agent and drilling fluid systems such as temporary plugging drilling fluids that utilize calcium carbonate and deformable particles as a core treating agent and temporary plugging drilling fluids that utilize alkali soluble micrometer-level cellulose as a temporary plugging agent have been developed in China and foreign countries. However, owing to the non-homogeneity of low-permeability reservoirs, different protective materials are hard to match with low-permeability reservoirs effectively and their reservoir protective effects are not satisfactory. There is an urgent need for a novel protectant and drilling fluid system that is suitable for low-permeability reservoirs.
Existing drilling fluids can essentially meet the requirement for cuttings carrying and well wall stability, but their low-permeability reservoir protection effects are not ideal; consequently the reservoir core permeability recovery rate is low. Though reservoir protecting agents are added in the drilling fluids, the characteristics of low-permeability reservoirs are not taken into consideration fully. Therefore the reservoir protection effect is not ideal and filtrate intrusion into the reservoir often occurs, resulting in damages such as water sensitivity and water blocking etc.